The present invention relates to a gas detecting sensor, particularly to a sensor for detecting the partial pressure of oxygen gas contained within exhaust gases discharged from an internal combustion engine of an automobile or the like and measuring air to fuel ratio of the combustion mixture to be supplied into the internal combustion engine.
Recently, "lean burn system" that is, the system of operating an internal combustion engine with an air to fuel ratio larger than the stoichiometric ratio so as to reduce harmful components contained within the exhaust gases and lower the fuel consumption has been proposed and employed practically.
The above described "lean burn system" requires a detecting means for accurately detecting air to fuel ratio in a range of lean mixtures. U.S. Pat. Nos. 3,933,028 and 4,012,709 disclose examples of such a detecting sensor as described above.
These detecting sensors are provided with a sensing element formed of cobalt monoxide (CoO) or an alloy of cobalt monoxide(CoO) and magnesium monoxide(MgO).
These detecting sensors are further provided with a heating means so as to surround the sensing element for maintaining the sensing element at a predetermined temperature, for example 900.degree. C. in order to prevent cobalt monoxide from changing into tricobalt tetroxide (Co.sub.3 O.sub.4) and to compensate the temperature change.
However, the above described conventional detecting sensor has such a problem that it is difficult to maintain the temperature of the sensing element at a predetermined temperature without being affected by the change in the temperature and the flowing volume of the exhaust gases.
Generally, the electric resistance of the sensing element changes due to the change in the temperature of its environment and the change in the chemical constituent of the exhaust gases. Therefore, when the temperature of the exhaust gases is changed, the partial pressure of oxygen gas cannot be precisely measured so that the air to fuel ratio of the combustion mixture cannot be precisely controlled.
Furthermore, in order to maintain the temperature of the sensing element at a predetermined temperature without being affected by the change in the temperature and the flowing volume of the exhaust gases, the control circuit having the complex structure is required.
Accordingly, one object of the present invention is to provide a gas detecting sensor for precisely measuring the partial pressure of oxygen gas contained within the exhaust gases when the temperature of the exhaust gases is largely changed or when the sensor is positioned in the oxidizing atmosphere or the reducing atmosphere.
Another object of the present invention is to provide a small sized gas detecting sensor having a simple structure, by which the partial pressure of oxygen gas can be precisely measured.